The present invention relates to hydraulic cylinders in general and more particularly to hydraulic cylinders made of a plastic molding and having a molded-in metallic annular insert. Hydraulic cylinders, such as hydraulic brake master cylinders and master cylinders and slave cylinders for hydraulic apparatus operating motor vehicle mechanical clutches, are sometimes made entirely of an impact and burst resistant plastic molding, but are more commonly made of a housing molded of plastic and provided with an internal tubular member or sleeve defining the bore of the cylinder in which a piston reciprocates.
When molded of plastic, the cylinder housing is generally provided with a bore increasing slightly and progressively in diameter from one end to the other of the bore, because of the molding technique requirement that internal cavities of a molding be provided with at least a small amount of draft to facilitate unmolding. The bore of the plastic housing is subsequently provided with a thin wall metallic sleeve, preferably made of steel, which provides a smooth and substantially cylindrical bore surface, of the same diameter from end to end of the bore, and in which the piston, including the piston seal, is slidably disposed. Such a structure requires that, in addition to providing appropriate sealing from the ambient, more particularly at the pressure end of the cylinder, means be provided for retaining the metallic sleeve in the bore of the plastic housing to prevent longitudinal motion of the sleeve relative to the housing. One end of the sleeve, preferably the pressure end of the sleeve, may be installed in abutment with a portion of the housing, and the other end of the sleeve, at the open end of the cylinder through which the piston rod projects, must be provided with a retainer for holding that end of the sleeve in position and with a second retainer for preventing the piston rear end from being driven out of the cylinder under the action of hydraulic pressure or of the piston return spring. Conventional metallic retainers may be used for that purpose, such as a plain spring split ring engaged in a groove formed in the internal surface of a plastic housing or, in the alternative, such as a split spring ring or wire ring provided with radially extending lugs engaged in slits formed in the internal surface of the housing. An inconvenience in ring/groove assemblies is that the plastic material forming the wall of the groove tends to shear under load or stress and, in the alternative structure consisting of a split ring with lugs engaged in slits in the internal surface of the housing, installing the ring with the lugs engaged in the slits is a difficult and time-consuming operation and forming the slits in the housing results in an expensive mold and in a delicate unmolding operation requiring retractable cores.